Abrasion resistant furnace lining



Patented Nov. 5, 1940 UNITED STATES I 2,220,101 .mnssron aasrs'rm masonLINING Raymond C. Benner and George 1. Easter, Ni-

tion of Delaware No Drawing.

Application March 20, 1937,

Serial No. 132,081

6 Claims.

This application, which constitutes a continuation in part of ourcopending application Serial No. 721,065, filed April 17, 1934, relatesto furnace linings and materials used in their construction; andparticularly to the linings of rotary furnaces which are subject to bothchemical erosion and abrasion by hard particles at high temperatures.Outstanding examples of such furnaces are cement kilns, rotary limekilns, and various calciners.

In the lining of high temperature furnaces, such as rotary cement andlime kilns, it has hitherto been found that erosion of a bare.refractory lining is relatively rapid due to a combination of chemicalaction and mechanical abrasion by the charge passing through thefurnace. Efforts have been made to counteract this effect by using alining which would not react chemically with the charge. In this way ithas become customary to use high alumina firebrick for the lining ofsuch furnaces. This has not however been entirely satisfactory for tworeasons: one is that the ordinary bonded high alumina material continuesto shrink when subjected to high temperatures, with the result that veryobjectionable cracks develop in the lining, and eventually pieces of thelining drop out.

Attempts have been made to get away. from this shrinkage by calciningand even fusing the alumina before bonding it to form blocks or rammingcements. The calcining orfusing of the material has overcome theshrinkage, but the second objection remains, i. e., that the bondbetween fused particles of the material breaks away and the lining failsmechanically just as in the case of the unfused refractory, causing theface of the furnace lining to wear back comparatively rapidly. Amodification of this type of lining in which relatively large lumps ofprefused alumina have been set in a matrix of refractory cement hassimilarly proved unsuccessful.

In order to combat the difficulties described above it has becomecustomary to line cement kilns, etc. with a rather open, soft bauxitebrick having material porosity. The initial portions of the charge ofcement then penetrate these pores and form an anchorage for thesubsequent part of the charge which adheres to the penetrative portions.The final result is a self-lined kiln in which the walls are coated witha sintered semisoft layer of the cement or other material being treated,over which layer the main body of the charge passes. Such liningsareseriously damaged if the kiln is allowed to cool off, and are at bestnone too satisfactory for use with lime (01.266-43) or with thehighermelti n'g ciaiaumamama cements which do, not start toj'si'nteruntll tem peratures sufficient to cause shrinkage of the lining areapproached. 'I'hemaximuin temperature of operation of kilns havingsuchlinings is close-1 1y limited as, if the charge becomes too fluid,the

lining is quickly worn back to 'the'oflmalbrick' 1 which then erodes orshrinks" andfalls out'as before. This type of lining and the necessarily.re-

sulta-ntmethod of operating the kilnh'ave'how- 1o ever been the mosteflicient heretoforefound and have come into wide use despite obviousdeflciencies. A further experiment hasbeen triediniwhich such kilns havebeen lined' iwith blocks'of cast .15 mullite. This material is composedof'aluminurn silicate containing f'rom 'to alumina. i. It however provedtoo'in' permeableto'permit operation of the kilns in the"customary'nlannerand was unsatisfactory whenoperated at higher tem 20peratures on account of the rapidityyof erosion of the surface of therefractory'by the chemical and abrasive action of the charge.'Spallingdifliculties'jwere also encountered;

'We'haveifound it possible to largelyliovercome 25 these variousdifflculties by lining suchfurnaces with extremely hard, highlyrefractorysolid cast masses consisting essentially of crystallinealumine. with only minor percentages of. impurities.

The solid masses are not'subj ect to the breaking ,30

out of particles by disruption of the bond between them as a result 'ofthe abrasive impact of the charge; and hence far 'outlalstlinings madeof the same materials in the form of bonded granules. They are,moreover, so refractory, inert and hard evenat furnace'tempera'tfuresastoalmo'st completely resist wear and erosion byYhot: lime or Portlandcement. 'Iheirjspall resis'tance, par'-, ticularly if about 5% ofmagnesia isialso present,

far exceeds that of othercast refractories. Such 40 cast alumina'refractories do" notsoften below 1850 to 2000 C'. and m'ay'beuse'd towithin 200 C. of those temperatures without premav ture failure. They'have'the further advantage.

that the material being processeddoes not pene- 5 trate them thusforming a layer of material having a different coefficient of expansionwhich is almost certain to break the refractory on cooling. Kilnslinedwith cast alumina blocks may therefore be shutdown and restartedsuccessfully, a1- 59 though very slow change in furnace temperature,

is desirable if this is to be done successfully.

Either the alpha variety or the betavariety of cast alumina is suitedfor refractory linings vof this type. A cast lining of beta alumina isparticularly useful where the furnace lining is sub- :lect to alkalivapors. It is also somewhat more resistant to temperature changes thanthe alpha variety. It may be desirable, on account of cost, to usecastings composed of slightly impure alumina containing minorpercentages (perhaps of magnesia, lime or soda. A small percentage oftitanic. or silica may also be present without detrimentally affectingthe usefulness of the refractory material. It is in general desirablethat the casting be composed of at least 90% A1201, although so long asthe crystal phase is chiefly alumina the performance is fairlysatisfactory even somewhat beyond that point. umina for the productionof castings for our purpose may be fused inan electric furnace such asthe submerged are furnace described and shown in United States PatentNo. 929,517 to F. J. Tone; and the operation of the furnace may besimilar to that followed in the production of synthetic aluminaabrasives.

in general, the furnace will consist of a water cooled iron shell havingno other lining than that built up by the material being fused as it isfed into the furnace. Fusion is effected initially by the heat from thecarbon train between two or more carbon or. graphite electrodes insertedin fed in and the electrodes raised as the fused mass is built up.

When the material has arrived at the proper temperature and the correctdegree of fluidity, the, casting is made by pouring the molten materialinto molds of the desired shape and size. The furnace may be adap edeither for tapping the molten material out through its side or for tilt-0 ing so as to pour over a lip. The molds may be of granular refractorymaterial'bonded with a core binder such as is commonly used in foundrypractice, or may be made of slabs of prebumed refractory, or of asuitable metal, or of carbon. 45 The molded articles should beof theproper shape to form a section of the furnace lining, and the sizeshould be as large as is convenient for a bricklayer to handle in liningthe furnace.

If desired, the molds may be preheated and are 50 insulated to preventtoo rapid loss of heat by embedding them in a molding flask in whichthey are surrounded by sand or other heat insulating material. The moldsshould be provided with risers of ample size to permit complete fill- 55ing of the mold without interference by material freezing in theheaders. Moreover, the headers should be of sufficient size to allow forthe contraction of the molten material as it solidifies in the mold. Ifthe riser is made wedge-shaped 60 wi h its minimum section immediatelyadjoining the mold, removal of the excess material constituting a headeris facilitated. After a. mold is filled it is moved away and additionalmolds filled successively.

65 The molded sections are left in the mold for heat treatment or,particularly in the case of iron molds, are taken from the molds shortlyafter the outer walls of the casting have solidified. Thereafter theyare carefully cooled by any of the methods well-known in the art; andafter they are cool any objectionable remainder of the header or otherminor roughness is removed by chipping or grinding.

This method of casting forms no part of the present invention and isrecited only to clearly distinguish from the process known asslipcasting" which is sometimes used in the fabricaafter ready forimmediate service without fur-' ther firing, although due to the closedense strucurs of such refractories it is desirable that the temperatureof the furnace be brought up relatively slowly in order to avoid,spalling.

While it is entirely feasible and from a service standpoint is desirableto make theentire lining of the furnace of cast alumina blocks, theseare relatively costly so that it is within the scope of our invention touse them for lining only the hot zone or other portion wher their use isparticularly indicated by the greater severity of the serviceencountered. Similarly, the upper end of the kiln may be lined with betacastings to withstand alkaline vapors and the heat shock caused bycontact of the cold charge with the hot lining, while a. lower part islined with the denser alpha corundum castings, which' are mechanicallystronger and very resistant to the semimolten Y charge.

We claim:

1. In a lining for a rotary lime or Portland oement kiln, a zone formedof dense, hard castings composed essentially of crystalline alumina.

2. A rotary lime or Portland cement kiln lining comprising denserefractory castings laid with substantially no cementing material between them, said casting being composed of at least 90% alumina.

3. A lining for a rotary lime or Portland cement kiln, comprising a zoneconsisting of dense non-metallic refractory castings, which castings arcomposed of at least 90% alumina.

4. A lining for a rotary kiln comprising a zone composed of beta aluminacastings and another zone composed of alpha alumina castings.

5. A lime or Portland cement kiln lining composed of dense refractorycastings consisting essentially of crystalline alumina.

6. A lining for a rotary lime or cement kiln consisting of denserefractory castings composed of at least 90% alumina. and comprising aminor percentage of an alkaline oxide.

RAYMOND C. BENNER.

GEORGE J. EASTER.

